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Shouldn't the instructor in this scenario try to avoid a deadstick landing?
In the Netflix drama "The Crown" (season 1, episode 4) Prince Philip has flight lessons with Group Captain Townsend (below is an image of that scene, I was unable to identify the plane).

After Prince Philip complains about the noise of the engine, Group Captain Townsend deliberately turns it off, and they both enjoy the silence for a while. After some chat, Prince Philip notices the ground getting closer:

Prince Philip: "Shouldn't we get her started now? The ground seems terribly close."
Group Captain Townsend: "No, it's too low to restart, sir. We'll do a dead-stick landing."

And they land safely.
My question is: if that was a real event, shouldn't Group Captain Townsend (as the instructor) interrupt the relaxing gliding with a warning? Something along the lines of "excuse me, sir, we're losing altitude, if you want me to restart the engine we have to do it now", or even "I'm restarting the engine now". I mean, shouldn't he be checking the limit altitude for restarting the engine?
I'll not ask why Group Captain Townsend didn't do that, it would be off topic, opinion-based and, despite the Group Captain being a real person, that scene is fictional anyway (The Crown is not 100% historically accurate). I'm just curious about the most probable behaviour of a real instructor in a situation like that.

Is your question "what is the lowest altitude to attempt and engine restart on the DH82?". If so, you can edit the title to reflect it, because it is more about engine restart than about landing.

@ManuH no, that's not my question. My question is why did the instructor wait until the apprentice warned about restarting the engine to say "it's too late".

OK understood. Thus your question is about the instructor's responsibility, isn't it? Thus you may edit the question to reflect it as it is more about instruction method than about landing.

The scene is pure TV with no basis in reality I'm afraid. The Tiger Moth has no electric starter, and must be hand propped, so as soon as the engine stopped they were committed to landing in a field. Prince Phillip would have been well aware of that.

@GdD Like a modern jet or prop engine, if airspeed is sufficient, the propeller will windmill fast enough to allow an unassisted restart. Here, they delayed restart until they were going too slow and too low to dive.

I agree, once the prop stops it's unlikely you'd get is spinning again @user71659, even with lots of altitude.

@GdD note that Prince Phillip should have been aware of that. No guarantee he would have been aware of that. ;)

Just as a practical matter, you aren't going to be enjoying the silence in an airplane like that.  It may be quieter with the engine off, but there's still going to be a lot of wind noise.

Surprisingly, given the usual high level of knowledge on here, nobody has corrected the "Captain Townsend" error. The RAF officer Peter Townsend, who instructed Prince Philip, and had a romance with Princess Margaret, had the RAF rank of Group Captain. British and British Commonwealth air forces (and certain others) have a different set of rank names from normal army ranks. RAF ranks were created in 1918 and partly based on naval rank names. A Group Captain is equivalent in rank to an naval Captain or an army full Colonel, NATO grade OF-5. The rank is not shortened.

@MichaelHarvey Thanks, edited.

I have a feeling that an RCAF Squadron Leader became a "Major" in 1968 when the Canadian Forces were unified, and had to wear a cockamamie uniform and rank insignia until 2015/2016 when they at least got the RAF-style stripes back, if not (yet?) the rank titles.

if that was a real event, shouldn't Captain Townsend (as the instructor) interrupt the relaxing gliding with a warning? Something along the lines of "excuse me, sir, we're losing altitude, if you want me to restart the engine we have to do it now", or even "I'm restarting the engine now". I mean, shouldn't he be checking the limit altitude for restarting the engine?

This question appears to be predicated on the idea that the instructor had no plans to land the plane with the engine turned off. Having watched the scene, I'm far less convinced that this is the case. In fact, it's not until Philip remarks that the ground is coming up that the Captain tells him they'll be landing deadstick.
In the series, this instructor featured is a renowned flying ace with huge amounts of combat flying experience including a number of forced landings and ditchings.
Bob Hoover (who you could argue with cut from a similar cloth) regularly displayed and demonstrated with the engines turned off, up to and including landing in that configuration.* On the basis that turning the engine off was deliberate and not an emergency, I don't think the scene after that point is particularly unrealistic. They appear to be making a straight in approach and land on an airfield - it's entirely plausible the Captain made a deliberate decision to land with no engine from the very start.
*Example:

I believe you're correct: seeing that way, Townsend never planned to restart the engine... Had not Prince Philip asked about the altitude Townsend would simply do a deadstick landing without anouncing it.

@Megapteranovaeangliae I didn't add this to my answer as I couldn't cite anything, but my understanding is that early aviators routinely landed with the engine out. Now obviously that wouldn't be the case in the 50's, but I can imagine a WWII ace, having been taught by an earlier generation, being very comfortable with the idea. Obviously nowadays we value the safety margin and ability to go around far more

That scene is completely unrealistic as you have guessed. When the engine of a single engine airplane fails there's always a chance you may not be able to restart it, so the procedure is to slow to best glide speed, choose a field to land in, and start flying an approach. Next you work through your engine failure restart procedure, and if you can't get it restarted you make a mayday call and prepare for your emergency landing. You want to brief your passengers, stow away loose items and clear your exits, shut off the fuel supply to reduce the chance of a fire. Above all concentrate on flying a stable approach and sticking the landing. At some point attempting a restart becomes a distraction, so you stop trying and concentrate on getting it down safely.
So you'd never think 'this is nice and quiet...' if the engine quits, you'd think 'this is far too quiet!' and take immediate and decisive action. You would never sit blissfully enjoying the silence as that's a great way to die. This is why practice forced landings are a significant part of training, and pilots should continue to practice them.
Regarding that particular scene, the Tiger Moth has no electrical system, so no electric starter. It's got one gravity fed fuel tank, and very little a pilot can do if the engine quits. If it's something a pilot can fix quickly, for instance a lean mixture mixture or accidentally switching off the magnetos a restart is possible if the prop is windmilling, but only if quick action is taken. The characters would have been well aware of this, so the scene isn't realistic.

I have to disagree with this answer - I feel it lacks the context of the scene and makes a lot of presumptions about the characters motive. In the scene Townsend turns it off completely unprompted and the landing (on a runway) happens very shortly after - given his somewhat exceptional aviation experience, it's just as likely he entirely deliberately to land with the engine off. This wouldn't have been entirely uncommon at one point and his attitude to flight safety would have been very very different to modern flying training.

I should add, not to say the scene is "realistic" or that it happened like that

I think discussion of character and motive is getting off topic for this site @Dan. I have had the good fortune to talk to more than one former RAF pilots from that era, and none of them would say the approach to training was blase. The phrase 'you've just had an engine failure' was as common in their training then as it was now, probably more so as engines were less reliable, but the idea that a pilot wouldn't know a dead stick landing was inevitable or have already started to prepare for one lacks credibility.

I'm not sure I follow your point - with all due, have you tried to quickly watch the scene? There's no real insinuation that he didn't know exactly what was coming - to me, after a quick watch, it looks he's set up with the field in front and planned the whole thing. I think you and the OP are reading an emergency or something un expected into a scene which has no such thing. Now whether it's a good idea to turn the engine off - well, that's a different question. But all I see is a deliberate piece of decision making from an expert pilot. And Bob Hoover made an entire career out of that...

I've watched most of the series @Dan. Well, at least the seasons with that cast. Again, discussions of character, motivation and writer intent are out of place here.

If they're out of place, then the question is out of place as is your answer. The behaviour of most modern, real instructors would be to never deliberately turn off the engine. But in this series, the character did ad I don't see where the emergency comes from? Like I say, Bob Hoover does half a display like this - I wouldn't do it, nor would anybody I fly with but here we are!

The question asks what is realistic for an engine failure @Dan, the OP specifically states Townsend's behavior on the show is out of scope for the question.

But it wasn't an engine failure! What's realistic for an unplanned failure is a completely different question - the engine was deliberately shutdown.

I disagree with it not being realistic. My CFI shut off the engine on my first (“discovery”) flight, and we weren’t even within gliding distance of an airport. He did have me restart the engine, but only after we were on short final to land in a field if that didn’t work.

That was not smart on the part of your CFI @StephenS.

@GdD A few years later he got fired for doing it on a discovery flight with the chief pilot’s kid. But I wasn’t saying it was smart, just that it’s not unrealistic.

First, a real instructor wouldn't have turned the engine off at all. The engine is kind of important, after all.*
Second, you're never "too low" to start the engine. After all, you can start it when you're on the ground, and that's about as low as an airplane can get.
Third, to actually answer your question, yes, if there was some limit to how low they could go, then the instructor should have said something before reaching that point.
The airplane in the screenshot is a de Havilland DH.82 Tiger Moth.
*There are some instances of instructors turning the engine off to test their students on emergency engine-out procedures. But that's for that specific purpose, not just because they don't like the engine sound. Note that I'm not going to argue one way or another as to whether or not it's a good idea, just pointing out it does happen.

well, starting up the engine may take longer than it takes to hit the ground, which would make it too late to start the engine in that specific scenario.

I fundamentally disagree with this answer, at some point you have to abandon restarting the engine and concentrate on the landing.

Old planes like the Tiger Moth did not have electric starters; they had to be hand propped. There is an altitude/airspeed window where the speed of the windmilling propeller is sufficient to restart in flight. If you are too slow and can't dive to accelerate, then the engine cannot be restarted.

@GdD That assumes that the engine quit because of a problem with the engine itself. If they just turned the engine off, there's no reason they couldn't just turn it back on.

@user71659 That's true, I didn't consider that. However, if that was the case, the instructor should have said that they were too slow to restart, not too low. I've never seen the movie in question, but the OP makes it sound like its an instructor training someone to fly, in which case, clear communication is vital. (Of course, it's also possible that the OP just misheard the dialog.) Also, that would militate even more to the instructor not turning the engine off at all, due to the problems with restarting it.


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Is the ground clearance of the B737 engine low compared to similar aircraft?
I'm referring to this other Question.
I want to know more about the B737 and the distance between the engine and the tarmac.
There are several Incidents reported, on 737 damaging the tarmac.

Is the distance on a B737 engine really that little comparing to
other Aircrafts (for example A320)?
Is this a design fault? Also have a look at the "Jet-blast pressure study" page 13-14.

It's not a design fault, it's a feature. More ground clearance means an aircraft redesign, and no committee has the stomach for that. With the engine diameter growing with efficiency improvements, we slowly got into this and now try to make the best of it.

Is the distance on a B737 engine really that little comparing to other Aircrafts (for example A320)?

The actual distance depends on the engine installed and the weight and balance of the airplane at the time. But there isn't a huge difference between the two planes. Comparing the figures, the current A320 is about 10 cm higher than the current 737 NG. The A320neo and the 737 MAX will be pretty close, with the MAX potentially getting 5 cm lower.
737 (MAX): 43-58 cm (1.42-1.92 ft)
737 (NG): 46-64 cm (1.5-2.08 ft)
737 (CL): 46-53 cm (1.5-1.75 ft)
737 (100/200): 51-58 cm (1.67-1.92 ft)
A320neo: 46-56 cm (1.84 ft)
A320: 57-85 cm (2.2-2.79 ft)
Those reports also show exhaust velocity profiles at takeoff thrust, but differences in the diagrams make it hard to compare. The IAE engines look to have much lower velocities than the CFM engines on either plane.

Is this a design fault? Also have a look at the "Jet-blast pressure study" page 13-14.

It depends how you look at it. In the linked report, there are several recommendations for both blast pads and the operator's procedures, but nothing regarding the design of the aircraft. So rather than telling the designers to mount their engines higher above the ground, they are making sure that facilities and procedures are designed to prevent things like this from happening.
As far as being a "fault" unique to the 737, the study mentions:

...an accident very similar to that of G-DOCT, which involved an Airbus A320...

So it's not the only airplane that can cause problems. And the newest versions of both types will have similar and smaller distances, they probably decided that there is no facilities-related reason important enough to choose differently.

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How are emergencies handled during In-Seat Rest?
Consider a Boeing 737 cruising over the Atlantic when an engine flames out. The procedure here is clear: the pilot flying should continue flying the plane, while the pilot not flying should pull out the QRH and follow the checklist for engine flameout. What happens, though, if the pilot not flying is engaging in Flight Crew In-Seat Rest? I assume they're woken up, and then I see a few options:

the pilot flying continues to fly while the pilot not flying wakes up and overcomes their sleep inertia. After 20 minutes, they go through the engine out checklist
the pilot flying continues to fly while the pilot not flying immediately goes through the engine out checklist, despite the risk of sleep inertia
the pilot not flying immediately starts flying despite the risk of sleep inertia, and the pilot flying goes through the engine out checklist
the pilot flying flies the plane while going through the checklist as the pilot not flying wakes up

These all sound like awful options. So  what would actually happen in such a situation?
Related: Is a pilot allowed to sleep during a flight?

I imagine the Pilot Flying shakes the Pilot Monitoring by the shoulder and says "Sonny come to life - things are happening!"

I would think the pilot at the controls would undoubtedly continue flying.  The guy asleep would have no SA.  Even when I was learning how to fly, if I was at the controls during an emergency, it was my aircraft and I flew.

20min to wake up? I think that if the flying pilot really says "Sonny come to life - things are happening!", the resting one will be awake in less that 2 seconds, noticing later that 20 seconds would have been fine, too.

From experience I can tell you that in case of emergency sleep inertia does not last 20 minutes. Nature has ironed that flaw out eras ago.

@SHAF - SA? situational awareness? r u on a phn?

Do any aircraft include ops for resting pilots with only 2 crew?  And can 737 fly ETOPS greater than 60 minutes and thus trans-oceanic?

Option 2:

the pilot flying continues to fly while the pilot not flying
  immediately goes through the engine out checklist, despite the risk of
  sleep inertia.

When waking up to an emergency situation your body will start to produce adrenaline which gets you over the sleep inertia very quickly. If the situation is not clear to the PNF, he will be briefed by the PF.
Besides that, in a checklist flow, both pilots are involved and crosscheck each other's actions.

Also... it's a checklist, how much thinking do you really have to do? Read it, confirm it with PF, put your hand on it, confirm it again, do it, check it off, next item.

This doesn't really seem sufficient. Consider this incident, in which sleep inertia was apparently enough to confuse the pilot monitoring over a minute after waking up. It still seems like engaging the pilot monitoring in checklist activities immediately after waking up carries a significant risk.

The infamous Venus avoidance manoeuvre! That is actually the opposite of what you describe; the PNF awakes to a non-emergency situation, then perceives an emergency to which the PF does not respond and he acts accordingly, thereby creating an emergency.

@DeltaLima right, and it seems like a pilot woken up in an emergency will be even more confused, and more likely to make an error.

@JonStory consider British Midland 92, where an error in dealing with engine failure was deadly

@raptortech97 indeed, but tiredness wasn't at fault there, and the pilot guessed which engine had failed, rather than following a checklist... I don't see any connection. The same goes for the other incident mentioned: caused by tiredness, but not due to an inability to follow a checklist when tired. That's the beauty of a checklist, it does a lot of the "reasoning" for you.

Adrenaline and fast hands are far more likely to exacerbate an emergency than deliberate, methodical action.  Being woken up and thrust into an emergency could have disastrous consequences if the PNF makes sudden adrenaline fueled decisions.

@DeltaLima: Google turns up nothing useful for "Venus avoidance manoeuvre".

@RhinoDriver this line of thinking suggests to me that PNF (who was sleeping) should run through the checklist (which is collaborative and cross-checked) rather than be given control of the plane. which is, i imagine, what happens.

Aren't airline rules on in-seat rest such that the duration of the nap doesn't cause that tiredness when you awake?

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Why are critical flight computers redundant?
At least in airliners, the truly critical computers are redundant. Typically three identical copies of the autopilot computers run in parallel and compare results; if one computer disagrees with the other two, its output is ignored. The system allows some processors to be faulty while maintaining the operation of the overall system.
But why? I've never heard of microprocessors suddenly failing. Sure, there could be manufacturing errors, but those would have been caught at the factory. Perhaps the program (and its proof) is wrong, but it would be wrong in the same way across the processors. Similarly, bad input would cause bad output across all three computers. What kind of errors does this redundancy protect against? Do microprocessors sometimes just do math wrong?
If a microprocessor is overheated or overloaded and spontaneously fails, I would expect it to stop doing anything and produce no output. To deal with this kind of failure, you'd want to have a backup processor, but you wouldn't need to compare the outputs of three computers—any output produced would be assumed correct, so you'd be happy to directly use the output of any processor that was producing output.
Related: The answer to What is the purpose of multiple autopilots? simply says "redundancy" before going into how this is achieved.

I will wait for authoritative answers, but on the systems I have been involved with, the 3 computers ran different software, produced by independent teams and proven to generate the same outputs for the same inputs.

@Simon I know the Shuttle had backup software ("design diversity"), but Wikipedia claims this practice is becoming less common.

It might be, I've been out of the loop for about 20 years.  BTW, I'm a software engineer now and have seen processors fail and, more commonly, RAM chips fail.

@Simon But RAM can have ECC, right? In the worst case, duplicating RAM is a lot easier and cheaper than duplicating the entire computer. The processor failing is much more of a concern. Do you think you'd be able to write an answer about how processors fail?

My question is essentially the same as the question I just linked. Should I close this as a duplicate of the other, and then add a bounty to the other? Should I edit the other question to focus on how the redundancy is achieved, to better match the answer?

In my opinion, your question is valid as it is, and I'm interested in the answers.  As to how processors fail, it's not worth an answer.  I've seen 2 from memory.  One was a fan failure, and the chip just fried itself and the other was unknown.  Manifested itself as increasingly weird errors and blue screens followed by a total failure.  RAM will almost certainly have ECC but that can only correct single bit errors and report double bit errors.  If more bits fail, which is easy with a physical error, then ECC is of no use.

@raptortech97: Autopilot is not that critical; the plane can be flown by hand. The really critical systems are fly-by-wire controls. In Airbus they run on pairs of dissimilar boards (i386 and m68k) with independently written software that cross-check each other, these pairs are multiplied for fail-over and there is independent set for primary flight controls (elevator and aileron) and another for alternate (spoilers and horizontal stabilizer), so if either fails the other can still control both pitch and roll. I believe Boeing system on 777 and 787 is similar.

@JanHudec I agree that the autopilot is not usually critical, but a failure during a Cat III autoland is considered catastrophic.

I find the choice of using 3 to be a bit odd. In order to deal with one of them failing in an arbitrary way you need byzantine resilience, which cannot be achieved with less than 4.

@kasperd I might be wrong, but I think that's only when messages can be forged. With dedicated physical connections, you can't really forge messages.

@raptortech97 The analysis of systems without byzantine resilience assume that each node is either operating perfectly or has stopped communicating entirely. It only takes one single random bitflip to invalidate the analysis of such systems.

@kasperd that's not what in talking about. The type of Byzantine resistance analysis you suggest relies on the assumption that computers can lie and forge messages from other computers. The three-party system is solvable if you have cryptographic hashes to verify identity.

@raptortech97 It is only solvable if you assume that the faulty node stops sending any messages. If a single node fails in a way causing it to send inconsistent messages, you lose all guarantees.

@kasperd Let us continue this discussion in chat.

Have you not heard of good old Murphy? Anything that can go wrong will go wrong

Yes, microprocessors sometimes do maths wrong.

"I've never heard of microprocessors suddenly failing". That's because you are not familiar with electronics. Ask here and you'll be enlightened. In addition a flight computer is not made only of a CPU/MCU. Keyboard, display, connectors, memory, clock, other chips, other electronic components, power unit... name it.

This question is badly expressed - appallingly so. The enquirer appears to mean flight computers employ redundancy. But what he in fact asks is why are they obsolete?

Failure modes to consider:

Overheating. This changes the chip's timing properties and eventually results in error. This can manifest as single-bit errors in the middle of seemingly normal operation; it will eventually crash, but may output bad data first.
Water damage. Manifests as a parasitic resistance on the board and may cause you to misinterpret bits as high or low. May be leaky housings, condensation, etc.
Electromagnetic interference. (The system is supposed to be resistant to this, but it's still worth thinking about).
Physical connection problems. Either during construction (soldering faults) or induced afterwards (heat, vibration). Microscopic cracks in boards or joints can pass QA but result in intermittent faults. Again this can lose you a single bit at a time. This is related to the Xbox "red ring of death" issue.
Failure of other components. Capacitors are the usual suspect; electrolytic, tantalum, ceramic all have different failure modes. Again this may result in a system that mostly works but is prone to misinterpreting marginal values or suffers from timing drift.
Weird materials science ("Purple plague", tin whiskers due to lead-free solder)
Part QA may not be up to the standards you expect (suppliers shipping inferior parts with a bogus "aerospace grade" label). Hard to detect even after it's happened.

It's important to recognize that in high-speed digital systems you don't get nice clean "one" and "zero", you get a series of rising and falling edges that are smeared out by the parasitic capacitance and inductance of the wiring. This is inherently vulnerable to being misinterpreted in marginal electrical conditions.

Wow, thanks for the great detail! I do want to note that I find it hard to believe that the parts would not have been up to standard. In aviation, the original design and every spare part had to get FAA-certified, and there's a large paper trail tracking everything. The bogus spare parts industry is rather small nowadays.

I'm coming from the electronics rather than the aviation side here, so I'm not familiar with how well the paper trail works, but you might find this interesting if you like detail: http://www.bunniestudios.com/blog/?page_id=1022

(Anecdotally I think the majority of "worked in QA fails early in production" faults are mechanical failures of solder joints; I believe aviation/MILSPEC used to prefer wire-wrap for exactly this purpose, but that's no longer practical with tiny packages)

This is the best answer. The simple fact is that CPUs can produce wrong answers and often do when operated outside of their specified voltage/temperature/etc. ranges because it messes with the timings. A timing difference of picoseconds can cause you to read the wrong value off of a line. Similarly, a very small voltage difference can cause a value to be read as 1 instead of 0 or vice versa. This is why when people are testing overclocks, they run burn-in tests that do math as quickly as possible for hours on end and watch for wrong answers.

As far as saying it will eventually crash... it might eventually crash, or it might just keep on producing bad output. When CPUs get too hot, for instance, they often continue happily running their code all day long while producing wrong answers, especially if one or more ALUs or FPUs are operating out of spec, but the instruction decoder is not (which is not a terribly uncommon failure mode.)

And latchup. If from nothing else, then from cosmic rays (as RedGrittyBrick's answer mentioned).

As other answer pointed out: A CPU can fail. Either partially (giving erroneous answers), or totally.
Moreover all computer are subject to cosmic radiations that can once in a while flip a bit in memory (in addition to other sources of error like short circuit, ...). That's why scientific experiments and long running servers use ECC memory.
Spaceships also use specific hardened CPUs to limit this effect as they are less shielded from such interferences.
Planes fly at high altitudes and are subject to more of this interferences than your earth-bound computer.
Even if the event of this happening is very uncommon (but not unheard of), you MUST ensure that the results are 100% accurate.
One bit flipped could change the behavior of your plane in unpredictable ways, like inverting the controls, inverting the flight envelop protection law, ...

Not only can cosmic radiation flip a bit in memory, it can also flip a bit in the CPU. Now ECC for memory still makes sense as memory has far more bits, but the risk still exists for CPU's.

Yep, that was there is ECC memory AND Hardened CPUs :)

I think this is the most important reason. If bad memory were the only concern, we could just triplicate the RAM but have a single CPU. But if a cosmic ray flips the wrong bit in the CPU and the CPU isn't triplicated, the result could be catastrophic.

As comparison, there is more background radiation in a typical passenjer jet at cruise altitude than there is today at ground zero in hiroshima.

This is called a Single Event Upset. There will often be 3 copies of the same data stored in memory as well as another unit to take over in the event of a critical fault.

I know this question has already recieved a handful of answers, but none of them seem to address the issue of why there are three systems in the redundant set, rather than just two.
First off, as was pointed out by Simon, Jan Hudec and RedGrittyBrick, the designs aren't identical at all. Indeed, they are often completely different for very good reason: the probability that any given problem will affect all redundant systems, and especially affect all redundant systems in the same way, goes from "small" to "utterly miniscule bordering on nonexistent". Compare How dissimilar are redundant flight control computers?
Second, as to why there are three systems in each redundant setup. When everything is working fine and the aircraft is in steady flight, for some value and some given set of inputs, all systems report that a correction of 0 (of whatever unit) is needed. At this point there is no problem, and the computers just serve to maintain the present state. Now, one of the component systems fail to do its job properly for any reason, and start reporting that a correction of +50 units is required. That is, the set of responses changes from [0,0,0] to [0,0,+50]. Two systems agree and the third reports something else, so we can likely safely disregard the outlier and go with the two systems that report the same: treat the result as [0,0,incorrect] and ignore the incorrect result while logging technical details and displaying some sort of prominent warning that the systems need to be looked over ASAP. But what if we had only two systems to begin with, and one of the two fails in the same way? The determined correction needed goes from [0,0] to [0,+50]. Quick now: which value is correct? Should you maintain state, or correct by +50?
At that point, there is no way to know whether correcting by 0 or +50 is the proper course of action. You could take an average, but using an average of two numbers (one of which is likely to be incorrect) could actually be worse than either value by itself.
By adding a third system to the redundant set, you add a tie-breaker for the situation where there is one malfunctioning system. Only if two of the three systems start to malfunction at the same time do you have an actual problem, and if the aircraft is having such problems that two out of three redundant systems are giving erroneous outputs then you likely have some serious trouble to begin with.

Why are critical flight computers redundant?

Software
One point that has been missed is that the redundant systems are often independent designs, especially the software. This guards against design faults (or software bugs) that may otherwise cause problems under rarely occurring combinations of circumstances.
Hardware
Even if a microprocessor is highly reliable, there are a number of factors that may be relevant

aircraft fly at high altitude where the atmosphere provides less protection against cosmic rays. This not only affects crew health but has the potential to interfere with electronic devices.
Avionics systems are composed of more than just microprocessors, there are surely also other, more failure-prone, devices - such as capacitors. There are innumerable ways electronics can fail, e.g. vibration induced failure of grounding leading to interference on data lines (e.g. from analogue sensors).

I've never heard of microprocessors suddenly failing.

Reliability ≠ Safety

Many accidents occur without any component “failure”

Caused by equipment operation outside parameters and
  time limits upon which reliability analyses are based.
Caused by interactions of components all operating
  according to specification.
Highly reliable components are not necessarily safe

From Nancy Leveson, MIT, via UCSD

"I've never heard of microprocessors suddenly failing." I had one fail in a desktop once. I'm typing away at whatever, and the screen just went black. After the usual testing we sent it back, they said we needed a new CPU.

People who've never heard of CPUs failing have never gone through the AMD Athlon/Pentium4 days where frying a CPU wasn't uncommon

Thanks for mentioning that indeed the software is not identical, but different teams write for different hardware, but to the same specs. The original question is misleading, to say the least.

Most of the answers have revolved around the potential for computer hardware errors and things of that nature. While all that's true, no one has mentioned what the computers are actually looking at.
Let's say you're on approach, getting ready to do a CAT III autoland, and you only have two computer systems. Both computer systems compare the #1 and #2 radio altimeter systems. Only there is a malfunction with one of the radio altimeter systems causing a discrepancy of some arbitrary value that is not within limits.
How does the computer know which one is wrong? One computer looks at radio altimeter system #1 and sees 500 ft. The other looks at system #2 and see 1000 ft. Which one is right and which one is wrong? How could the computer possibly make that decision?
Enter the third computer. If the value of what it sees is commiserate with that of any of the other two computers, it can effectively "vote" the invalid reading "off the island."
I should note that most of these computers have anywhere between two and four processors all comparing their own results. That is the INTERNAL redundancy to avoid hardware failure, but having numerous cross comparisons of external systems is largely the reason a third system exists.
Note: As an A&P mechanic, 9 times out of 10 it's one of the external systems having failed (radio altimeter, MMR/ILS miscompare, etc...) that causes a degradation in capabilities - NOT the computer itself.

Computers fail, spontaneously, all the time. You're not used to that because you've not used many computers. But consider somebody like Google, who runs massive data centres containing thousands of computers. The software that runs Google is designed around the explicit assumption that computers do fail because it happens multiple times per day. Now, an aircraft doesn't contain very many computers but the ones it does contain are safety-critical. So they're duplicated to make sure their failure doesn't cause a problem.

I think the OP was especially asking about the fact that there are three which compare instead of just two in the case the hardware dies.

From everything I've heard about Google's systems, they're designed to handle the total failure of any computer, not computers that are misbehaving

Hmm. I think dealing with malfunction rather than outright failure is implicit in my answer but you're both right that it's not very well tailored to the question. I'll have a think and either improve or delete it.

Google's systems are built around fail-and-retry as that's built into all the Internet protocols. Other large systems are similar and even embrace "crash-only" operation (see Netflix "chaos monkey"). This isn't appropriate for safety-critical real-time systems. It's an interesting contrast between a cheap system which in practice works nearly all the time versus a much harder to develop and more expensive system which can offer design guarantees.

The chance of processor failure is very low indeed, but not zero. Upon failure of the processor, what happens during the transition time between failure, and full functionality after re-boot?  With a rare event like this, could we ever build up enough experience to be sure that we've tested in all circumstances? We're talking about < $10^{-9}$ numbers here.
Backups are prone to hidden failures. The backup is normally not used and only switched on when needed - but has something rusted through, or has a funky mould nestled in a comfortable warm spot and cause a short upon activation? Murphy still haunts aerospace applications. The backup can be tested before take-off, but what if it shakes loose and the main processor fails? The chances are slim, but all major accidents nowadays are caused by unlikely strings of events like this.
Redundancy is useful because it continuously demonstrates that the main devices are functioning properly, and it is used for flight critical circumstances. Back-up systems may be used if you can do without the main device, or if it is guaranteed that the back-up will always work, like manual actuation of flight controls.
An autopilot in cruise is not flight critical, and may be disconnected without grave consequences. In a CAT III landing where the runway can only be observed once you're driving on it, they are absolutely vital. You don't want the autopilot to disconnect 10 metres above the runway, no visibility, gusty side wind - there is no time to engage the back-up.

I appreciate the answer. Note that I use they/them pronouns, not he/him.

I've noticed and amended.

"An autopilot in cruise is not flight critical, and may be disconnected without grave consequences", indeed true in theory, but has been once lethal when coupled with airspeed indication failure.

@mins In reality each event like autopilot disengagement during cruise is given a safety level, and has to meet that safety level.  The level of redundancy and rigor is adjusted to meet the safety level required. Autopilot disengagement during cruise is serious, yes, but not "catastrophic" and so is marked during safety analysis as only a "minor" or "major" problem.  The same safety analysis may mark autopilot disconnect and airspeed indication failure as a joint issue with higher consequences if there's a significant interaction between the two.

On specific redundancy, the installation environment of these systems is probably the biggest factor.  Not only are many systems crammed closely together in tight spaces, but airflow is often very limited in there.  Heat is a great destroyer of many a microprocessor.  Airplanes also vibrate, a lot, due to spinning engines, in-flight turbulence, and simply landing.  Poor solder joints and sub-par crimp jobs or a loose connector backshell, and you got a bad connection, or worse, an intermittent one.
On redundancy in general, if you experience a BSOD at work, comparatively it's no big deal.  You might have lost the document you were working on, but that's about it.  If aircraft systems go out, you have a real problem.  It's difficult to achieve, but the redundancy is there because the lives of hundreds of people are relying upon it.

If we look at this from a strictly engineering standpoint, microprocessors like anything have a cycle lifetime. Generally speaking it's very long, and the PC you are posting this from most likely will be out dated long before it hits is cycle life time. Although a microprocessor has no moving parts it does take input from various sensors. I can only assume that the inputs are fused in some way, but that does not mean that a spikes are completely eliminated and isolated if they do occur. For what it's worth, even relatively small surges will fry a microprocessor. Keeping that in mind, to err on the side of caution multiple systems are used. With the ever shrinking size of technology it has gotten easier and cheaper to carry a spare so from a sales standpoint the peace of mind is there. Again it's better to have it and not use it than to not have it when you need it.
To directly address your question, I have been around microprocessors, micro-controllers and the like for a long time. In that time I have had maybe two or three fail spontaneously, usually related to heat. In a plane this may not seem like an issue but actually extreme cold can cause issues as well as extreme heat when it comes to electronics. That being said I have toasted countless units by hitting them with overloaded inputs. Let's say your plane got hit by lighting (I know that modern airlines are protected against this) but for arguments sake let's say a ground was bad: this would easily toast a unit.
Side note: It is more common for memory chips/drives to fail these days. This is something you may never know as most modern computers can deal with dead memory be it in the disk or in system memory.

The avionics will generally be in the avionics bay below the cockpit, or in the cockpit itself, so the extreme cold won't be an issue, but with inadequate cooling they could overheat. What I didn't clarify was that if a microprocessor spontaneously fails, it should be possible to detect that and switch to a backup. The system in use compares the three outputs, implying that a microcontroller could produce an incorrect output, as opposed to simply failing to produce any output.

I find the statement that "most modern computers can deal with dead memory" dubious at best. Maybe in the strict sense that "if a memory cell goes bad, it won't immediately cause the computer to crash", but it will cause erratic operation as soon as that memory is actually used for something. The saving grace in a way is that in modern PCs, the lion's share of RAM is not actively used; it gets used for cache. Which can be equally bad if you have no way of detecting a problem (which in practice means you're using non-ECC RAM, which most desktop systems don't).

That is incorrect, PC's are capable of skipping over and ignoring bad sectors on disk see here http://en.wikipedia.org/wiki/Bad_sector likewise the linux kernel now supports badram and badmem which are capable of telling the system to skip over corrupt addresses, which is what I was referencing. You are however correct that bad memory can cause erratic behavior, however it is not always the case.

@user16230: And who tells the kernel to exclude that memory? certainly not the program that is currently running off from that memory. This is a purely offline last resort measure that is done after detecting that a memory bit is defective by other means than a program currently running on it. Even for bad sectors the result is data loss. Nothing that can be fixed mid flight by reflashing the system. Also bits might misbehave under the weirdest conditions only, that can not be detected by even the best memory test programs. Or why do you think rowhammer was only discovered recently?

Again I agree however I was merely pointing out that it is possible to deal with bad memory (not that I advise doing it in flight).

@user16230: You are still not getting the point. It is not possible to deal with bad memory, if you have no idea that it is bad, or will get bad in 5 minutes. You need means to detect bad memory, recover stored data and remap. This is not only far more expensive than having three redundant systems, it is also totally impossible for all failure scenarios. As such dealing with memory failures mid flight might be possible, but you have to plan for the events where it is not possible, by implementing majority vote decisions, which have a far lower rate of error.

@PlasmaHH you could presumably RAID a few memory chips together and connect them to single CPU without the expense of having three memory chips and three CPUs.

@raptortech97: you stil need three ram modules, otherwise you have no majority vote and could at most detect that one bit went wrong. Yes you could create a custom system that uses a hamming code or other perfect higher order code, but what then? You still have possible bits that could flip in registers or caches. Hamming code these too? what to do with bitflips in the decoder? Also this would cost an awful lot more than just majority redundance. And it won't deal with things like analogue read out errors (e.g. higher impedance from sensor to adc)

If a microprocessor is overheated or overloaded and spontaneously
  fails, I would expect it to stop doing anything and produce no output.

Have you ever overclocked a CPU, or watched an old piece of hardware die?  You can get all kinds of weird artifacts while the cpu is still running.

CPUs used in safety critical tasks are paired with a watch dog, a (simple) system similar to railway industry dead man switch. This stops/resets the CPU as soon as it is not able to do a predefined handshake (e.g. to discharge a capacity before it is fully charged)

In an airplane, safety is more important than any other factor (after that comes optimum weight for fuel efficiency, and overall cost is third). If airplane would not be safe, not enough people would fly, and the airline industry would collapse. That's why there are FAA regulations, and that's why there are so many rules for the airlines. (the airport security check is another subject, related to national/political security with immigration, etc, so when we say 'safety' of the airplane, I mean engineering-wise)
Critical systems on board (i.e. systems that are required to fly the airplane) will need redundancy. Like the burner in the jet engine has 2 igniter, even though one is enough. Also, if an engine fails, the other engine can fly the airplane, and the computer will compensate for the left/right force imbalance. Many systems in the airplane rely on the computer so it needs to have a 'plan b' (redundancy is one of the 'plan b')

This doesn't answer the full question. I know that most things in aviation are redundant, but all of that redundancy has a specific reason - some mode of failure that could make the redundancy useful. I was asking what mode of failure chip redundancy protects against.

Because, despite the theory being good, the reality is that not all computer components are equal.
A specific example from the early 90s: Intel produced the 486/33 CPU (it was quite cutting edge and blazingly fast for the day). Most left the factory just fine, but some had an esoteric bug in the FPU that would generate incorrect answers. The magazines of the day were filled with calculations you could put into your spreadsheet that would produce X if your CPU was good, or Y if it had the bug.
If your plane happens to be running with one of the faulty CPUs in it, and just the right set of data happens to be collected and fed into any of the flight control programs and runs into this FPU bug, you'll be happy that the other two CPUs calculated the correct value and kicked the errant one out of the loop.

I think your example is misleading. First, I think you're actually talking about the Pentium FDIV bug. That was a design error, not a random manufacturing error. Every processor that was shipped had the bug, until the design was corrected. In that case, redundancy woudln't have helped: you'd just get multiple copies of the same wrong answer.

It's either the Pentium FDIV bug as mentioned by @DavidRicherby, or something like the 80386 double-sigma issue. I don't think the 486 ever had the sort of problems described in this answer.

@MichaelKjörling It sounds a lot like a conflation of the 386 double-sigma (affecting a seemingly random sample of manufactured units) and the Pentium FDIV bug (floating point and lots of public attention).

Regardless, it'd be be cheaper to thoroughly test every processor than to triplicate them.

@raptortech97 Tell that to NASA. They don't know about it.

@MichaelKjörling that's about testing designs for radiation tolerance, not about running each production sample through basic operational tests

@DavidRicherby, the OP might be conflating the Pentium FDIV bug with the origin of the 486SX, where 486DXs with a faulty FPU had the FPU disabled and were sold as integer-only CPUs.


text:
Why am I getting unusually high CHT on ground operations on one engine in a Seneca II?
Aircraft had sat for 3 months, and on startup the right engine CHT (Cylinder Head Temperature) was climbing noticeably higher than the left. After taxiing and run up CHT was near top of green. Redline is 460. Other engine readings are normal. Annual inspection completed, mechanic said compression is ok and CHT still in green. I had the probe changed, but the problem persists. During flight, the reading was quite high compared to left engine and compared to what it used to be. What could be the reason for this?

Does fuel flow match between them? How do the engines compare when you go to lean them out?  The problem here is that, never having seen your airplane, folks in internet land can spitball possible causes, solutions, and troubleshooting all day long, but none of it is worth much since we can't see what you're seeing. This sort of question doesn't tend to be a great fit for the Stack Exchange format, and may well end up being closed. Your (or another) A&P is probably your best resource.

If the gauges are good (swap them to see if the problem moves to the other engine), the baffling is good (baffles must be tight to the fins to force air between them with no leaks around the plenum), the cowl flap works normally (not being unknowingly blown partly closed by loose linkage), the most likely cause is an intake leak that's causing the cyl with the probe to run lean.
Either the rubber intake hose is bad, the gasket at the cylinder is leaking, or the sheet metal intake tube to which the hose connects, that is swaged into the intake spider tubes at the oil sump, is leaking.
This is an old problem on Lycomings. The little sheet metal pipe stub  is installed with a rolled bead that engages with a female groove in the hole in the sump, and depends on interference for a seal.  They can lose the interference and start to leak (you will be able to move it if you loosen the rubber hose).  There is a (very expensive) swaging tool (I believe you can rent from Lycoming) that re-rolls the bead from the inside.  Some people prefer to just remove the sump and send it somewhere to be done.
Although the redline is there, you really want to keep CHTs below 400F if at all possible.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What type of BITE is involved after installing a new display on an aircraft?
There are 3 types of BITE (Built-In Test Equipment):

power up
interruptive
continues

which one of these is being used after installation of a new display on an aircraft?

You should indicate what makes you think only one type of test is run.

Are you asking about a commercial aircraft getting a replacement screen, or are you asking about new avionics? Are you interested in GA installations?

First, let's clarify the terms.
BITE (Built-In Test Equipment) is the hardware and software within a product that performs BIT (Built-In Test).
There are three common forms of BIT:

Power-on BIT (PBIT).  This is run every time the unit powers up. Depending on the unit function, some units have two variations of PBIT; A comprehensive check performed after a cold start, and an abbreviated check performed after a warm start.  Warm starts typically occur in-air after short power interruptions. The abbreviated PBIT is intended to get the unit back online as quickly as possible. PBIT only reports faults.
Continuous BIT (CBIT).  Generally implemented only in critical functions, CBIT provides a continuous (or near continuous, periodic) test. CBIT is a background test and like PBIT, only reports faults.
Initiated  BIT (IBIT).  This BIT is only run when it is initiated by the crew or maintenance personnel. It will typically take the longest time and be the most thorough BIT that can be performed. IBIT will, upon completion, provide a status report.

So to your question; when a new display is installed (or any other unit is replaced) and power is restored the unit will perform a PBIT and potentially begin CBIT (if it exists). After the unit is in normal operation, the technician that replaced it will normally perform an IBIT and verify the unit passed all tests.
It's important to remember that BIT is intended to identify hardware faults. If you are concerned with 'glitches' due to flipped bits (caused by single event upsets) or similar, that is typically addressed using integrity monitoring. This is more common than CBIT and it will only flag 'bad data' as it can't specifically identify what caused a fault. It only identifies that data is suspect as the primary computation and the monitor disagree. A hardware failure will generally result in the integrity monitor providing a continuous fault condition, but it's primary purpose is to identify 'soft' or intermittent faults.

All three of these types of test will likely be conducted when a new display is installed in to an aircraft and they would happen each time the aircraft is powered on.
The only difference between a new installation and an existing installation is that the BITE might raise a problem with the software loaded. If an aircraft has three displays and one of them is replaced with a new one, it might have a different version of software loaded on to it. The BITE might contain a compatibility check to ensure that all the displays are running the same version of software.


text:
Is there an official English version of LTF-UL?
I have found the German version of the ultralight regulations under Germany airworthiness authority, but I have not found a link for English version of that. is there any?

I had translated it all in 2019, while working on the "certification" of an ultralight. I can see if I find the files next week (first week of March).

For LTF-UL 2019 there is no official English translation, unlike, say, documents from the United Nations or the Vatican.  German aviation forums have plenty of statements like

Ja eine offizielle englische Übersetzung gibt es tatsächlich nicht
English: Um, yeah, actually there's no official English translation

But the document's language is technical and precise enough that Google Translate can manage it accurately.

Edit: in 2020 Google Translate worked, but I've lost my copy of that translation, and in 2024 Google Translate generally translates only the easy parts and leaves technical words in the original German, even obvious ones like Ultraleichtflugzeug and Hängegleitern.  Worse, what it does translate is littered with gems such as

The fact that the air transport vehicles are safe now that ultra-lightweight vehicles are being used
means that they must be able to travel safely and that they must also be able to speak.

and

The safety of the air transport is in the vicinity of the safety equipment and the safety shield is
visible, with the maximum safety guarantee for the safety of the safety equipment.

"English: Yes, there is actually no official English translation" which is actually confusing and sounds like a machine translation. "English: No, there is actually no official English translation" would be a much more natural English wording. And far less self-contradictory.


text:
Which radar altimeters are susceptible to 5G C-band altimeter interference?
There is apparently a great deal of consternation over the pending expansion of 5G

The emergency petition claimed that interference from the C-Band "will cause irreparable harm and jeopardize the function of critical aircraft safety systems, which in turn threatens to divert or cancel thousands of flights every day, thus disrupting millions of passenger reservations, causing substantial disruptions for air crews, further interrupting the U.S. and global supply chains, and eroding the safety margin that the industry and the Federal Aviation Administration have worked so hard to achieve."

Apparently the FAA wanted two more weeks to study the issue

As part of this proposal, we ask that your companies continue to pause introducing commercial
C-Band service for an additional short period of no more than two weeks beyond the currently
scheduled deployment date of January 5. During this time, the FAA will identify the priority
airports, issue the required Notices to Air Missions (NOTAMs) and begin approving AMOCs.
During this time, the FAA will review information relating to the size of the buffer zone around
critical airports and will seek to reduce the size when safely able based on data from aviation
manufacturers. The FAA will make every effort to complete this work as expeditiously as
possible

AT&T and Verizon rejected that request

The FCC had compelling reasons for [limiting the 5G spectrum being used]. Spectrum interference disputes
typically involve simultaneous transmissions on the same frequencies. But radio altimeters do
not operate on, or anywhere near, the C-Band frequencies. Rather, they operate in a frequency
band (4.2-4.4 GHz) that is separated by at least 400 megahertz from the C-Band frequencies
(3.7-3.8 MHz) that AT&T and Verizon will begin using in 2022 and at least 220 megahertz from
any C-Band frequency authorized for use in the future. This helps explain why C-Band 5G
service and aviation operations already coexist in nearly 40 other countries where C-Band
spectrum has been deployed without any negative impact on aviation.

What equipment is actually being threatened by this rollout, that would cause airline companies to file suit over it?

Are you asking about the aircraft or the equipment that are [possibly] affected? The equipment is the radio altimeter; the aircraft are any aircraft that employ radio altimeters, particularly in conjunction with CAT II/III ILS approaches.

@randomhead Good point. I'm not entirely sure. I would assume some aircraft carry affected equipment, but I'm not sure which to ask about specifically. If you have suggestions for improvement, I'm open to them.

This is really about the modulation and filtering electronics on the radar altimeter.  Many aircraft will be flying with replacement electronics.  Some details: https://en.wikipedia.org/wiki/Continuous-wave_radar

Here are two of the recent statements from the FAA the earlier of which states In 2018, the Air Line Pilots Association raised concerns to the FCC.   and goes on to link this statement which references this study and ultimately their concerns were about interference with radio altimeters, with which many aircraft may be equipped. The concern was summed up with:

In 2014, working paper ACP-WG-F30/WP-14, “Preliminary Study into Radio
Altimeter Adjacent Band Compatibility, 4 ” was presented at ICAO. This
study detailed an analysis of the interference on radio altimetry
systems that could be caused by International Mobile
Telecommunications (i.e. wireless broadband). The study concluded
that, “there is the potential requirement for nearly 64 dB of
[Out-OfBand] isolation at the band edge to properly [protect] the
receiver from false altitude errors.”

The study is only 9 pages long and the conclusion is quite similarly worded but you should take a look if you are curious.
Lots of modern airframes use radio altimeters and an exhaustive list would be impossible to put together as they can be added to just about any airframe. The simplest answer is "a lot of commercial airplanes". I have even seen some larger GA airplanes equipped with them.
Practically speaking this creates an issue when flying approaches where RA is required. See this article for more info on that.

I would just note that according to Tom Wheeler at https://www.brookings.edu/blog/techtank/2021/11/22/will-5g-mean-airplanes-falling-from-the-sky/ the FCC agreed with the Boeing mitigation proposal to prohibit 5G within 4.1-4.2 GHz and in fact doubled that buffer to 220 MHz at upper 5G 3.98 GHz.


text:
Can you use a home built helicopter for solo hours?
I'm curious to know if anyone has heard of or has any knowledge of using a kit helicopter for the FAA required minimum solo hours of flight training.

I assume that this is for a Private Pilot - Helicopter license. I also assume you mean something like larger kit aircraft, not ultra-light helicopters like the Mosquito.
You can do this, but there are other conditions that must be met:

The aircraft must have completed it's flight test phase (this is usually the first 25-40 hours of operation)
The CFI must be willing to provide instruction in the aircraft. It will be difficult because either the CFI is not familiar with the aircraft or insurance won't cover them, some just won't fly homebuilts.
Your aircraft insurance must allow primary flight training in the aircraft (not many do)
The aircraft must meet requirements such has having a radio, transponder, etc for being able to operate in required airspaces.

The big ones will be finding a CFI willing to do this, and getting insurance to cover you. The insurance alone may outweigh the cost of renting a certificated aircraft.

Great post. I can imagine finding a CFI might be daunting. I know of several fixed-wing CFIs who will instruct in the client’s own plane. Even if that plane is an experimental. The comfort level is the key. Although, I have heard stories from my time-building friends of instructing in student owned airplanes that they eventually had to refuse due to being uncomfortable with the aircraft. And, these were type certificated aircraft with valid and complete EAROWCs. I know very few rotary-wing CFIs to ask about the same issue.

@DeanF. The CFI's I know will only instruct in a homebuilt if they have "time in make/model". Some are easier than others, a lot are comfortable climbing into an RV, but would be reluctant to so something like a Long-EZ. Homebuilts can have their own challenges to fly so many instructors I know won't do primary in them.

Insurance is indeed very important but it's also optional. In that sense it's not a hard requirement and if someone wants to fly without it they can, although I assume it would make finding a CFI even more difficult!

In the US, there is no requirement to use a type certificated aircraft to log hours for aeronautical experience. Nor is there a requirement to use a type certificated aircraft for logging instruction. Although, the aircraft can not be rented to nor by you. The aircraft would have to meet all the requirements for an experimental aircraft.

"the aircraft can not be rented to nor by you". Did you mean "nor by the instructor"? Most students are renting the aircraft so I'm not sure what you mean here.

@Pondlife - As an experimental, the student can not rent the aircraft to another person. The aircraft can not be rented to the student nor the CFI by another person. The CFI can not own the aircraft. Any one of these would make it a for-hire aircraft. However, the student may own the aircraft and have the CFI instruct them. It may be possible for a non-affiliated third party  to let the student borrow the aircraft for no compensation. I don’t think it would work if the CFI were the one borrowing the aircraft. That last area would be rather dark gray.


text:
What is takeoff power?
I Am asking about the takeoff power here not how the aircraft takes off.
What do I need if I want to calculate the power required by an aircraft to take off?
What parameters should I consider?

Welcome to Aviation.SE! I edited the question to (hopefully) make it more clear. If you wanted to know something else, feel free to edit again.

I believe you mean 'thrust' instead of 'power'. If so, this question will answer your question.

First you need to accelerate to take off speed within the runway length. Then you need to climb quick enough without decelerating. These two are the dominate factors for engine output at takeoff.

In the physics realm $Power$ = Mass x Acceleration x distance/time
To many it means "how much throttle" , which is how much fuel/time.
"Throttle" x prop efficiency = $Thrust$
This gives you a unit of Force that can be directly compared to glide data as follows:
Potential energy Mass x Gravity x altitude loss = $Thrust$ x horizontal distance flown
This rearranged to $Thrust$ = Mass x Gravity x altitude loss/horizontal distance flown
$Thrust$ = force required for straight and level flight, expressed in "pounds or Newtons"
So if your plane weighs 10 lb and glides 5:1, you need 2 lb of thrust for straight and level flight.
One would at least double, or even triple this for "takeoff power", depending on your performance preferences.  Sufficient "takeoff power" will give a safe climb rate to avoid difficulties with downdrafts.
For example:  my plane glides 5:1 at 60 mph or around 5000 feet/minute and loses 1000 feet/minute.  At 10 lbs it was calculated to need 2 lbs of thrust for straight and level flight.
How much more thrust do we need to climb 500 feet per/minute.  Again, the glide data provides a very good estimate, 2 lbs x 500 fpm/1000 fpm = 1 lb additional or 3 lbs total at 60 mph.  Higher speeds with more drag,  or lower speeds "behind the power curve" will require more than 3 lbs of thrust.


text:
How many smoke detectors does the A340 cargo deck have, and how are they wired?
I am a student of Aerospace Engineering, currently I am one year away from completing my degree. For my Electric Routing class I was asked to recreate in Catia the fire system in the cargo bay of an airplane and obviously justify why I made the decisions, for now I only have to decide which model I'm going to work with, its specifications, etc.
Looking for information on the internet, I decided that I will work with the Airbus A340 Lower Deck Cargo Compartment Class C. Doing a little research I understood that when there is a fire in this bay, from the cockpit the area is sealed and a percentage of Halon is released 1301.
But 2 questions arose that I would like to know:

How many and what type of fire sensors does this family of aircraft use (the type or model would be of great help)?
Does anyone know how this system is "wired" in general? I mean, in series, in parallel, which is better? How does the smoke sensor connect to Halon pumps?

This is information that I do not known where to find and I do not know if it is classified by the company and I need it to do my Catia model using the electronic Harness Discipline. For now I will be dedicated to modeling the part of my fuselage with the low cargo bay.
P.S: If you know of any regulations that I could read to better understand this topic, I would greatly appreciate it.

If you are doing this as a class project you are supposed to have learnt it from the said class right? It's explicitly asks "why you made the decisions" not airbus....

Hi and welcome on avaition.SE. This is not a forum, this is a Q&A website. Take a tour for more information. If your question is "how is smoke detector wired on A340 lower deck class C?", the title should reflect it (don't hesitate to edit your question to improve it). See the help center for more information.

@user3528438 Maybe the question was edited after your comment, but I think it's fair to ask now. I don't know where this information could be found either!

@user3528438 I agree with your assessment 100%. Sounds to me like the instructor is asking for the students' thoughts on the subject, not for them to research the how & why the manufacturer did it, then duplicate. I don't think this is a question that we can answer, as we'd essentially be doing homework for the OP. The "where do I look" portion is a reasonable question, but that's still OT at SE sites.

I’m voting to close this question because this is a homework assignment where the instructor seems to want the student's interpretation, not a regurgitation of what a manufacturer has done.

@FreeMan I'm one of the most-anti-homework people here, but I believe that open-ended and interesting questions should be judged on their own merits. There is a difference between seeking genuine knowledge that will help you do your assignment, and merely looking to tick a box. This is not ticking a box. Without the mention of homework, this would've made for a suitable question. Note, also, that the assignment is to route the electrics; the answer won't complete it.

In that case, @ZOMVID-20, it should be edited to remove all references to homework.

@FreeMan It could be better that way, yes, but we don't outright ban homework. Note that the homework is to do actual engineering drawings in 3D. They OP is not looking for ready models, they're asking for broader knowledge of how many detectors to wire up. This falls far short of doing someone's homework for them. If I were their instructor and saw this question, I'd find no reason to hold it against them, no deficiency in their performance, it's normal knowledge search in preparation for an assignment.

While I normally VtC almost immediately any "do my homework for me" type questions, I find I am leaning more in agreeing with ZOMVID-20's reasoning here, this coupled with Aircraft Design and Manufacturing being listed as explicitly on-topic, and I will VtLO.


text:
Airline Ground Operations via ACARS
I read about the ACARS "In Range" request pilots can send to request cabin services, lavatory services, wheel chairs, assistance for unaccompanied minors, etc. What about other services such as catering and potable water; those were not shown in the "In Range" request page. Do pilots have to make those request via radio or are those request handled by dispatch?
Any insight into how all this works (who is requesting what service) would be appreciated.
Also, is the catering company providing the potable water as well or are those two separate providers?

That'd depend on the airline, the airport, and the handler.
I'm only somewhat familiar with how it's done where I work, which is probably not the way it's done everywhere.
We have a central computer system that keeps track of all ground movements of aircraft and will alert the handler responsible for the operating airline of a flight as to which gate/parking the aircraft is going to be serviced at. The handler can then go out and provide the services contracted for that flight by that airline.
I guess the aircrew can request additional services by radio or through the gate telephone system through the ground crew as well if needed, but that'd be outside of the reach of the system I work with so I have no details as to how that'd work.


text:
Can the load factor during glider winch launch be measured with the G-meter?
I am looking at the flight envelope of a glider, the velocity - load factor diagram. I read that generally the air-frame, the G-meter and the pilot 'experience' the load factor as a G force (an acceleration).
My question is whether that is always correct. I think of a winch launch and believe that in this phase, because of the cable, the the G-meter and the pilot do not 'experience' the load that the air-frame has to deal with. Or in other words, there is load on the air-frame which is not due to acceleration. But I do not clearly understand the physics of the launch phase.
The question is important to me because if my believe is correct then during launch the pilot cannot judge the stress on the air-frame by the G-force he/she feels.

You are absolutely correct.
Since a string can only transmit tension forces (and no shear or compression force), the direction of the tow cable near the glider directly indicates the direction of its force. In the late phase of a winch launch the tow cable is pulling mostly downwards and has only a small forward component left. As long as the ratio between forward and downward component is still larger than the gliders lift-to-drag ratio, staying in the launch still adds energy to the glider, so a glider launch ends only when the cable shows an almost vertical direction.
All tow cables have a weak link at their glider end which limits the loads than can be transmitted. The picture below (source) shows a collection of color coded weak links with different limit loads (note the different center hole sizes), the strongest one being 10,000 N, mostly used for two-seaters.

The picture below (source) shows how the weak link is integrated: It sits in a U-shaped sleeve and is connected to the rest of the cable by shackles. On the right you see the rings which engage with the hook on the glider. In this particular case two weak links are used, and one has oblong holes at both ends so it can act as a fallback if the first snaps from some sudden jerk. Most tow cables have several of those weak links at their end and the appropriate one is selected depending on the glider type.

As you say, the load on the tow cable must be added to the weight of the glider to determine the lift force the wing has to produce in the late phase of a winch launch, which can easily be twice the weight of the glider for a total of 3 g. All while a g meter in the cockpit will read only less than 1 g (less because the glider is still climbing, with a nose-up pitch attitude). The pilot can only guess the load on the airframe, judging from indicators such as the wind noise, the stick force or the airspeed instrument. Another indication would be the sudden jump the glider performs when the cable is disconnected when still at high load. That happening, however, depends on the style of the winch operator.
When sitting in the winch on the ground, this load can be felt: During the launch the whole winch (normally a truck with a second engine and spools mounted at its back) will be lifted up a bit and drops back into its suspension when the tension on the cable is released. Still, this load is much less than the safe load the glider has been certified for which is 5.3 g for most modern gliders (see CS-22.337).
While the cable load is not dangerous, a more critical moment can occur during lift-off:
At the initial phase of a winch launch the force on the cable, acting in horizontal direction and at the bottom of the fuselage, creates a mostly horizontal acceleration and, in addition, a pitch-up moment which can stall the glider if not counteracted by elevator lift. Especially gliders with a fully-flying horizontal tail suffered crashes when their tail was still stalled during lift-off due to an excessive angle of attack on the tail. Therefore, it is important to keep the stick neutral and only push it forward when the glider raises its nose too quickly. The worst is to pull too early: A low-level stall will follow with the risk of an unsymmetrical wing stall, followed by a half roll and violent ground contact soon after.
If you are interested in the physics of winch launching: In 2008 a thesis has simulated the launch of an ASK-21 and describes what happens in detail.

Great answer, the need for the weak link becomes obvious. The figures in the thesis give a good insight into what is going on. I am happy to see that what my instructor teaches me agrees with the theory.

See also the safety leaflet from the British Gliding Association (shorturl.at/uDQY3) written for glider pilots. Interesting what they say about 'EXCESS SPEED NEAR THE GROUND (page 9).

I do not understand the bit "when their tail was still stalled" because the wing is supposed to reach the critical AoA earlier than the tail plane.

@EKA: On a full-flying tail (stabilator / all-flying tail) with extreme deflection (up or down) and at the low Reynolds number of the initial take-off acceleration, the stall can persist to a point where the wing creates enough lift for flight. Pilots need to keep the stick neutral until the glider lifts off, then the tail will work as intended.

I've never taken one of these weak links apart myself, but I was told that the two weak links are not actually in parallel. One of them is supposed to have a slightly longer hole so it only takes load after the first has failed. A quick image search seems to confirm this: http://www.air-store.eu/WebRoot/Sage/Shops/AIRStore-LuftfahrtbedarfundAvionik/5642/243B/AA37/1B41/6DE7/0A0C/05BA/78B6/gsobild_1005.jpg

@wrtlprnft: Agreed, here this is called "reserve-insert system": https://www.tost.de/katalog/tost-weak-links/?lang=en

@wrtlprnft: You are right. I had been under the impression that one yields a bit until both share the load equally, but the Tost page confirms what you say. However, I can assure you that I used tow cables with two links in parallel for two-seaters. But that was long ago.

I fully get the concept of the weak link, however, I don't understand the implementation shown in the picture above. It appears to me that the silver metal between the bolts will take the stress, not the yellow link plates. Therefore the "weak link" is the silver steel(?), not the yellow. Maybe I should ask a whole new question.

@EKA An all-moving tail similar to the Alphajet, Cherokee, or F-4 can be stalled before the wing given the right conditions.

@FreeMan: At one end of the sleeve (the silver metal thingy) there is not a hole, but a U-shaped dropout. The shackle is free to move out if it were not held back by the weak link. I admit, the picture does not reveal this. But it is the best I could find.

Thanks, @PeterKämpf, that makes much more sense and is completely invisible in this image.


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Why don't piston engines feather on shut down?
When turboprops shut down, they feather the propellers. If piston aircraft did this, would it ruin the oil system? Also, why do pilots feather very quickly when doing the engine run-up?

It is wrong to say “When turbines shut down, they feather the propellers”. Not all turboprops are feathered during shutdown.

"On many variable pitch propellers, the blade pitch can be increased to the point that the chord line of the blade is approximately parallel to the on-coming airflow. This process is referred to as feathering."

Hydraulically operated propellers feather the same way on both recip and turboprop airplanes, by dumping the oil supply that drives the blades in the fine pitch direction to the sump so that coarse pitch pressure moves the blades to feather. There is often a supplemental hydraulic pump to make sure there is pressure to do that following an engine failure, although in normal operation it isn't needed (counterweight props will rely on the counterweights + a spring that helps the blades move to feather).
Turboprops are feathered at shutdown mostly to stop them from windmilling in the breeze while not running, there being little resistance to rotation coming from within the engine. They'll rotate a bit if there is a breeze from the side pushing on the flat of the blade, but nothing like the pinwheel you'd have if they were set at normal operating pitch when stopped and there was a decent breeze coming from in front or behind. You don't want this happening when the gas generator part of the engine is not producing oil pressure for the turbine shaft bearings.
A piston engine doesn't need to do this for obvious reasons so the feathering system isn't implemented at shutdown.
Pilots won't work the feathering system on a piston engine run up; they just run the propeller control levers between max RPM and min RPM to exercise the pitch change system and get warm oil into the propeller hub, same as a non-feathering propeller. It may sound like it's feathering with the big downward whoosh, but it's just running from normal fine pitch to normal coarse pitch.


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When modeling a shear flow for dynamic soaring, is the coefficient always a constant?
Is a wind shear model for dynamic soaring always given by the velocity vector field:
F = < cy, 0 >,
where c is constant?
I've tried modeling c to introduce a decay in magnitude of the wind shear as it moves from left to right, but apparently this violates conservation of mass of the flow (assuming the flow is incompressible) and that the coefficient should be kept constant.

The shear layer is conventionally described as a "thin" transition above stationary air and below constant ambient wind.  Within that layer,

The wind's gradient w.r.t the altitude between the shear boundaries can be modeled by any smooth (say, twice differentiable) monotonic function.

-- Section III, "Novel Approach to Dynamic Soaring Modeling and Simulation," Kai et al, J. Guidance, Control, and Dynamics 42(6), 2019, https://doi.org/10.2514/1.G003866.
This paper mentions nothing about wind velocity changing w.r.t. horizontal location.  In fact it says

the wind blows horizontally by assumption

Because of incompressibility, and because the flow field is uniform both above and below the shear layer, any horizontal change within the shear layer would induce a vertical component of the wind vector.  (Handwaving argument: imagine a brick wall downwind in the layer.  That would force a downdraft along the wall.)
So today's models of wind shear ignore second-order effects like horizontal decay of magnitude of shear, perhaps because that suffices for the still interesting case of an albatros mid-ocean, as opposed to the turbulence immediately downwind of a mountain ridge.  But this won't be the case forever, of course.


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How to make a cockpit LCD display night vision compliant?
I am trying to understand what does it means by making an LCD display compatible with Night Vision? The aircraft is an old Mirage of 1950's.
If I use an LCD display in it's cockpit display unit then how would I make the LCD compatible with Night Vision capability?

Do I need to select a specific color (wavelength or frequency on spectrum) to display the data on the screen?
Or do I need to put some filter sheet on top of the LCD screen to make its Night Vision compatible?
Or does it mean something other than these?

Edit:
The only info about the OEM display is follows:
"The display is an ac TFEL flat panel display combined with contrast enhancement bandpass filters to achieve both sunlight-readability and night-vision google compatibility"
I want to replace this TFEL display with an LCD display. But I dont understand how will I do night-vision google compatibility in an LCD?

What is the standard or regulation you want to be compliant to?

Can you name some of them as I don't know of these, but I can work them back wards?

@GdD I have added more details in the post related to your query.

I don't understand this, are you building a custom display or avionics system? Do you plan to use night vision goggles?

I want to replace the TFEL display with an LCD display of a cockpit display device. The TFEL display is NVG compatible.. but I don't know how will I make an LDC as NVG compatible. Do I need to use a specific RGB color on LCD display to write the data on the LCD screen OR do I need to put any extra layer/filter screen on top of the LCD display to make it NVG compatible?

You can make an LCD screen NVG compatible by adding a filter screen on top of the display that complies with MIL-STD-3009.  You can get a copy of MIL-STD-3009 from several commercial sources or direct from the Defense Logistics Agency website.
There are a few companies that manufacture the filter screens. I would start with a search for NVIS filters.

The parameter is 'sunlight readability'. What makes an LCD fulfill this criterion? Is there some min 'nits' value for a cockpit display to be 'sunlight-readable'?

I don't know if there is a magic number for sunlight readability. It's a factor of brightness, contrast ratio, and the reflectivity of the front glass. You need low reflectivity to cut the glare which obscures the display. You might find some more info in the civil display standard, SAE AS8034.


text:
How to know the material of key-top?
Posted it here originally.
There are keys (push buttons) on the front panel of an avionics device that fits inside the aircraft cockpit. I need to know the material of which those key-tops are made from.

I can guess the material is some type of clear transparent plastic and then its painted with black color. The text on it is un-painted key surface. There is green light under the keys (buttons). That green light glows the text (due to transparent plastic) on the key-top as green in color.
This cockpit device is from a very old aircraft model of 1950's so we can say it is not using any modern high performance thermoplastic or composite material. How can I best identify the material of these key-tops?
Edit: How do I know if the key-top material is compatible with NVIS (Night Vision Imaging Systems)?

Can't put my finger on it, but I have serious doubts about these being an original item from a 50's plane...

I’m with the guess for double shot molding from Nguyen, with colored plastic. This is also used for some keyboard switches. Painting or even coating switches lead to horrible results after some time and these look still fine ;)

What plane are these from? The keypad is  similar to ones seen on IRS (or INS) panels, and a rather close match to F-18 Hornet's UFC keypad...

@Jpe61It's a mirage plane.

@scico111 original earliest 50's Mirages did not have panels with such keypads, they will be from a later cockpit update.

The part looks injection-molded. The choice of transparent, moldable polymers available in the 1950s is manageable. The most likely candidates are:

PVC (back then the most widely used polymer, but normally not used in injection molding)
Celluloid (unlikely due to its high flammability)
PMMA (plexiglass)
Polystyrene

PVC can be detected when a heated copper wire is pressed into it and then again held into a gas flame. The copper will react with the chlorine of the PVC and color the flame greenish (Beilstein test). The last two candidates can be easily identified by the smell they produce when molten. Styrene in the case of polystyrene and derivates of benzene in case of PMMA. Since PMMA is rather soft, it can easily be notched but also has low mechanical strength. When dropped on a table, PMMA parts will create a more muffled, lower frequency sound than parts made of polystyrene.
Both are similarly well suited for being painted. Some plastics like polypropylene and polyethylene are unsuitable for painting because of their low surface energy. But while already known in the early Fifties, those materials were not yet used for parts production. The best substrate for painting would be ABS, but similarly while being known back then, transparent ABS types suitable for molding were not yet used.
My guess would be polystyrene, but given the few hints I am not really sure.

Can Polycarbonate (PC) be used for the keys? The original keys may or may not be made of PC but what ever material those were made from will it be a safe choice to use PC material for the keys?

@scico111 While PC would be a good candidate from the late 1960s on, in the 1950s it was still a lab curiosity.

so if i am free to choose from PVC, PMMA, ABS, Polystyrene and PC then which one is the best candidate for the front panel buttons?

@scico111. Clearly, that would be ABS because it shows the best adhesion with the paint. Transparent ABS is now well established (while ABS was first made in 1946, I very much doubt that it would had been available in a transparent grade in the 1950s).

How do I know if the key-top material is NVIS material?

@scico111 What is NVIS? Night Vision Imaging System?

Yes, just like NVG's (Night Vision Goggles) etc. In the cockpit the LED lights behind the buttons should not interfere with the NVIS. What material of a key (button) will make it compatible with the NVIS in the cockpit? The key (buttons) have LED lights under them which will glow the text on the buttons.

@scico111 Wouldn't that depend on the particular system? The supplier should be able to answer this, I can only guess. Or is there a standard that I don't know of (maybe a MIL-Spec)?

Give the part to a machinist who works with plastics. He will touch the part to a grinding wheel, melting the plastic and making smoke. By smelling the smoke and looking at how the plastic melts, he will be able to tell you which plastic compound the part is made of.

Sometimes dropping it on a table is enough, because the different moduli of elasticity of different plastics will result in a clicking sound typical for that plastic.

These must be blue Cherry switches...oh wait ;)


text:
What is the name of Control equipment?
I wanted to know the name of equipment that controls the control surfaces of an aircraft by taking commands from the pilot through it's hotas stick. The aircraft could be a combat 50's or 60's a/c, so possibly there would be a specific name of such control equipment.
Will this equipment be the most safety critical equipment in such an aircraft compared to other equipment such as NAU (Navigation and Attack unit) or will there be any other more safety critical equipment in an a/c?
Edit: Let the a/c be a Mirage.

"HOTAS" is a concept of a functional stick (with buttons controlling equipment), it is not a universal name.

let the a/c be a mirage.

50s-60s aircraft were not HOTAS. Every pair of surfaces had its own linkages to the appropriate control or stick axis.

An aircraft like that would have used mechanical connections between the stick and the control surfaces.  Either cables, or push-pull rods.  According to this the Mirage III used "servo controls", which likely would have been hydraulic boost to the mechanical linkages.

Most aircraft of that era used hydraulic systems to actuate the flight control surfaces.  And yes, controlling the aircraft is critical to flight safety.


text:
To access EASA amc s or FAR gm?
How can I access EASA Acceptable Means of Compliance documents, or FAR guidance material docs ?

Depends on exactly what you need but the FAA makes pretty much everything available online. You can start here for most stuff and get directly to the Federal Aviation Regulations here. If you prefer paper you can take a look here.

Sir I checked your links burt still I couldnot find the Guidance Material which I heard about as a material to decide on means of compliances .

@denizcanelçi The FAA calls its AMC documents Advisory Circulars (AC). They're the first link on the Regulations and Policies page.


text:
Using aluminium busbar/cables in an aircraft instead of copper
I have just found out Airbus uses aluminium wiring in the A380 to save weight. However, I didn't see anything about whether all the wiring is aluminium, or if copper wiring is also used. What is the logic behind this, and do other aircraft manufacturers do the same? Does Airbus use aluminium wiring in aircraft produced since the A380 was introduced? Are there any risks associated with using aluminium wiring on aircraft?

It's my understanding that most aircraft wiring is aluminum because it weighs less and is almost as conductive as copper.

Already asked and answered here: https://aviation.stackexchange.com/questions/61510/is-aluminum-electrical-wire-used-on-aircraft/61513#61513

In smaller planes, AL has been used to connect the battery in the tailcone to the starter & other electronics in front of the pilot.  That has worked properly for years, with the main thing being to properly terminate the AL in crimped connectors with the proper 'grease' (I don't know what it is) to prevent any galvanic corrosion.  My plane has that, built in 1973 and still going strong.

These questions are closely related, but they are not duplicate. Instead they appear like "inverse questions" to me.

I don't understand how people say these questions are duplicate. I asked more than one question and still didn't get any answer to all of them. For example, "Does airbus use aluminium wiring aircraft produced since the A380 introduced" or "Didnt see anything about whether all the wiring is aluminium or if copper is also used"..

While it is true that copper is 1.6 times more conductive than aluminum per volume,

Conductivity in $\frac{\mathrm{MS}}{\mathrm{m}} = \frac{\mathrm{10^6}}{Ω\,\mathrm{m}}$:

copper:    56
aluminium: 36
  $$\frac{56}{36} = 1.56$$

aluminium is 2.1 times more conductive than copper per weight.

Density in $\frac{\mathrm{g}}{\mathrm{cm^3}}$:

copper:    8.96
aluminium: 2.7
  $$\frac{36 \cdot 8.96}{56 \cdot 2.7} = 2.13$$

Another important metric concerns the cost. The conductivity per cost is 7.1 times higher for aluminium.

Base metal price in $\frac{\mathrm{US$}}{\mathrm{lb}}$ (as of today):

copper:    2.71
aluminium: 0.81
  $$\frac{36 \cdot 8.96 \cdot 2.71}{56 \cdot 2.7 \cdot 0.81} = 7.14$$

Aluminium wiring is safe if it is sized and terminated correctly. In comparison with copper, aluminium is softer, shows more creep, and has 30% stronger thermal expansion.

That's great answer, I appreciate. "use copper where space is limited, and aluminium otherwise" Does that mean copper is usually preffered in fighter jets or UAVs regarding to space?

I don't know much about wiring in fighters. The statement is a theoretical one. In places where the cable concentration is very high, and the harness designer runs out of space, copper might help.

@huytergan outside of wiring, it means that landing gears generally use steel alloys where space is at a premium, and aluminium when weight is the main factor and the part can be as large as needed.

"There is no risk associated with using aluminium wiring."
I think you need to expand on that a bit. Aluminium wiring in houses has a very bad reputation (and has been banned for new construction in the US for some time) due to fires caused by gradual growth of oxidization leading to increased resistance at connections. This can be prevented or mitigated but doing so isn't trivial.

One of the factors limiting power transmission in aircraft systems where weight must be minimized is the type of insulation coating the wires. If the wire jacketing can withstand high temperatures, then the size of the wire (for any given conductor material inside it) can be reduced. the increased  I-squared-R losses heat the wire more but as long as the jacketing can take it, you're OK. this principle is commonly used in wiring for planes like fighter jets, where the weight of the wiring harness can be reduced by allowing it to run hotter.

I'd say there is quite a bit of risk associated with Al wiring. It just can be mitigated by being very thorough with it. For a long time, the rule was Al for thick cables and bus bars, Cu for most cables, as it's much easier to terminate. Only recently has Al been making more inroads.

@pericynthion I modified the statement with respect to your comment. I don't know much about the aluminium wiring crisis in the US, but my limited understanding is that due to lack of better knowledge craftsmen installed aluminium as a 1:1 replacement for copper. They didn't account for the additional cross section required and/or using terminals suitable for this material. I don't know if there was a generation of planes that suffered from similar problems, but I am confident that aircraft manufactures learned the rules of aluminium wiring a long time ago.

@Therac, yes no doubt. In direct comparison copper is more forgiving, aluminium needs better termination techniques.

@pericynthion That's the urban legend, yes.  When you look at the facts of the matter, it's actually copper that's the bad actor: copper terminals dislike aluminum wires but everyone else plays nice. Today, service panel lugs are aluminum because they play nice. The 1970s fiasco was due to UL hastily approving small-branch-circuit devices without sufficient testing. It is legal to wire your house today with Al, simply using AA-8000 wire and CO-ALR rated devices..  But no one will buy it, if you do, because ooga-booga.

@therac Techniques like using a torque screwdriver on just about everything, and fitting AFCI breakers on most circuits, are now Code for copper wiring, because of a long history of failures that look a lot like the alleged problems with aluminum wiring!

If aluminum is 2.1 times more conductive than copper by weight, and also ~3.5 times cheaper by cost-per-weight, shouldn't those stack?  Ah, I see it. In the third calc, where did 36 and 56 come from?  Since you are multiplying by cost per weight, they should be conductivity per weight.  In fact they are conductivity per volume.

@Harper, that brings the result to 7.1times cheaper  instead of 2.1, if you can agree with the  approach.

Yes, the approach looks right, and that conforms with my 12:1 back-of-napkin (easily explained by difference in market price) and 3.5-4:1 difference in practical cost of wire (explainable as insulation is a significant factor, in fact on small wires e.g. lighting branch circuits, mineral value pretty much leaves the room; there's little cost savings to aluminum wire for branch circuits).


text:
Where can I find details about a Cambridge Scientific Instruments Fuel/Air ratio Indicator?
In our restored Harvard Texan T-6 WW2 trainer aircraft, a Fuel/Air Ratio Indicator was installed. We cannot locate the wiring diagram of this system or information for the Cambridge Scientific Instrument Co. The instrument is P/N. 11622-1 Spec 94-27992. Please help us with the required information, or the indicator's pinout and functions.

The 11622-1 is a type A-7 single-engine exhaust gas analyzer. You can see it here on page 8. The instrument just tells you if the fuel/air mixture is too rich or too lean. Electrical inputs are simple; 12vdc and ground. Any competent instrument technician can open it up and see which pin is what. But that's only the beginning... the system includes tubes and an analysis cell, and the cell is probably pretty much useless by now, if it is still installed, but you can always try your luck.